Numerous types of silicone release coatings are known in the art. These materials are most frequently used in applications such as release paper for pressure sensitive adhesives (labels, floor tiles, self-adherent carpet, wall paper, etc.), caul sheets and casting sheets. In these applications the release coating is usually applied as a liquid and cured using heat or radiation to form the elastomer.
Radiation curable systems are those that can be cured by a source of ultra violet light (UV) or Electron Beam (EB). In electron beam curing, the silicone release coating is exposed to a beam of accelerated electrons in order to produce free radicals directly. Each electron produces a cascade of many free radicals as it penetrates into the sample. Penetration can be deeper than that obtained in UV cure and the substrate can be opaque to light or UV radiation, however, the presence an inert atmosphere is required. Two types of release coatings are common for electron beam curing. These two are acrylic functional and mercapto-functional compounds with the most common being the acrylic compounds.
The market for "release" products demands laminates with tight control of various specified levels of release force. This is to serve sophisticated product needs such as two sided differential release force tape used, for example, to secure insulating film to window casements. It is current practice to control release forces in thermal cure coatings by the introduction of different amounts of high release force additives into the release coating. It is often awkward to control the blend ratio of additive to coating. This makes it difficult to make minor adjustments in a release coating composition applied in a continuous manner to instantly change the release force performance when switching to a new product specification or when producing out of spec material.
Inert environments are required during electron beam curing to prevent the formation of reactive ozones, peroxides, hydroperoxides, and alcohols. These reactive oxygen containing species react with the organic groups in the coating resulting in the introduction of hydroxylic, carboxylic and other oxygen containing functional groups into the coating surface generally making the coating more polar. The hydroxyl, carboxyl or other groups present in the coating can interact chemically with the reactive ingredients in types of acrylic adhesives resulting in increased release force.
Nitrogen is the most common gas used for providing an inert environment in electron beam curing. However, it is difficult or impossible to obtain a reasonable priced source of nitrogen that is either totally free of oxygen or known to contain a constant level of oxygen. To obtain the lowest release forces, oxygen may need to be as low as 1 to 10 ppm while most commercial suppliers have been found to contain oxygen in the 50 to 200 ppm range.
Methods for providing an inert gas, other than nitrogen, suitable for use in electron beam have been developed. One such method requires the burning of natural gas in air to consume the oxygen. However, these methods result in impure gas streams containing water vapor, carbon dioxide, carbon monoxide and still a trace of oxygen whose level is difficult to control. They also require large volumes of natural gas and special furnaces to combust the natural gas. These methods are not readily used in large commercial applications.
It is an object of this invention to provide a mechanism for controlling the release force of the release coating without making composition changes during operation or using complex processes.